Wireless transmissions are used for wireless networking among computing devices as well as in other settings, such as to support broadcast of radio and television content. To avoid interference between the signals of different users, different portions of the full spectrum available for wireless communication are allocated to different uses.
The radio spectrum has been divided administratively into two primary sectors: the licensed spectrum and the unlicensed spectrum. The licensed spectrum consists of frequencies that have been licensed to organizations, such as commercial broadcasters, for exclusive use by those organizations. For example, a portion of the spectrum (also known as a “frequency band,” “band,” or “channel”) may be licensed to a cellular communications company for use in communicating information representing voice and data calls placed by its subscribers, or licensed to a media broadcaster to transmit signals carrying audio and video data representing television content. The unlicensed spectrum, on the other hand, has been allocated for free use by the public—though, typically, use of the unlicensed spectrum requires operating in accordance with some regulations, such as maximum power output regulations, that are designed to minimize interference between users.
The radio spectrum may be considered, however, to have a third spectrum sector: unused portions of the licensed spectrum. These unused portions may be categorized into several types. First, a frequency band may be licensed by an organization, but the organization may not be using the band at a given time, because the organization is not using the portion at any time or uses it intermittently. Thus, a first type of unused portion is a licensed band that is not in use or is underused, at a given time. Second, when frequency bands of the licensed spectrum are assigned to organizations, typically they are not assigned exactly consecutively; instead, there may be gaps between the licensed portions to prevent two organizations from interfering with one another. In a simplified example, if a section of licensed spectrum having a bandwidth of 50 MHz between 300 MHz and 350 MHz is to be divided between two organizations, the 300-320 MHz band may be granted to the first organization and the 330-350 MHz band granted to the second organization, with the 320-330 MHz band left unallocated. The unassigned band of the licensed spectrum, referred to as “whitespace,” is designed to provide a buffer between the two transmissions that minimizes the risk of a signal from the first organization interfering with transmission of a signal from the second organization. These whitespaces may be considered allocated in that they are located in a part of the spectrum allocated to television use, but unassigned in that they are not designated for use by a particular entity. Whitespaces are a second type of unused portion of the licensed spectrum.
Recently, it has been proposed that unused portions of the licensed spectrum may be used by unlicensed transmitters—acting as secondary users, where the licensee of the unused portion or of an adjacent channel is a primary user—as if the unused portions were a part of the unlicensed spectrum. Such use of the whitespace, however, is predicated on using techniques that prevent the newly transmitted signals (i.e., those signals transmitted by a secondary user) from interfering with transmission of existing signals associated with the licensees of the bands being used or the nearby bands (i.e., those signals which are being or will be transmitted by the primary user).
One class of techniques for preventing interference is referred to as Detect and Avoid (DAA), and has been implemented by some communication technologies such as the Ultra Wideband (UWB) protocol. In DAA techniques, a channel over which a transmitter desires to send information is monitored to determine whether data is being transmitted over the channel or if data is being transmitted in another channel that is close enough that interference is likely. Then, in accordance with the Detect and Avoid protocol, if communication is detected in the channel during the monitoring, the channel is avoided or only low-power transmissions are permitted. Using DAA techniques in this way prevents a new signal from being transmitted at the same time in the same channel as an existing signal—or over a second adjacent channel—and thus minimizes the risk that the new signal will interfere with transmission of the existing signal.
In whitespace, DAA techniques may be performed to detect signals from adjacent allocated frequency bands to determine whether transmission in the whitespace would interfere with existing signals in the adjacent frequency bands. If any existing signal from the adjacent bands is detected, then the whitespace may be avoided to prevent interference with the existing signal, or a transmission power may be lowered. In channels that have been licensed to an organization but are potentially unused, DAA techniques may be performed to detect existing signals from the licensing organization that may be present in the channel. If an existing signal is detected, then the channel may be avoided.